Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without the need to prepare printing plates. Amongst these are color laser printers that use the xerographic process. Color laser printers using dry toners are suitable for certain applications, but they do not produce images of a photographic quality acceptable for publications, such as magazines.
A process that is better suited for short run high quality digital printing is used in the HP-Indigo printer. In this process, an electrostatic image is produced on an electrically charged image bearing cylinder by exposure to laser light. The electrostatic charge attracts oil-based inks to form a color ink image on the image bearing cylinder. The ink image is then transferred by way of a blanket cylinder onto paper or any other substrate.
Inkjet and bubble jet processes are commonly used in home and office printers. In these processes droplets of ink are sprayed onto a final substrate in an image pattern. In general, the resolution of such processes is limited due to wicking by the inks into paper substrates. The substrate is therefore generally selected or tailored to suit the specific characteristics of the particular inkjet printing arrangement being used. Fibrous substrates, such as paper, generally require specific coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Using specially coated substrates is, however, a costly option that is unsuitable for certain printing applications, especially for commercial printing. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly and time consuming steps are needed to dry the ink, so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed “perfecting” or “duplex” printing) difficult, if not impossible. In addition, inkjet printing directly onto porous paper, or other fibrous material, results in poor image quality because of variation of the distance between the print head and the surface of the substrate.
In commercial settings, there exist additional printing systems, some relying on indirect or offset printing techniques. In such processes, an intermediate image of the final desired pattern (e.g., a mirror image) is typically formed on an image transfer member (e.g., a blanket or a drum) and transferred therefrom to the final printing substrate. The intermediate image can be, as in HP-Indigo printers, an electrostatic image produced on an electrically charged image-bearing cylinder by exposure of compatible oil-based inks to laser light, the ink image being then transferred by way of a blanket cylinder onto paper or any other substrate. Though such systems are better suited for high quality digital printing, the use of oil-based inks has raised environmental concerns.
The present Applicant has recently disclosed a printing process wherein inks having an aqueous carrier are jetted onto an intermediate transfer member (ITM) at an image forming station and dried thereupon before being transferred to the desired substrate at an impression station. Few systems implementing such process were disclosed, differing, among other things, in the number of image forming stations, the configurations of the intermediate transfer members, the number of impression stations, and the system architecture allowing duplex printing. More details on such systems are disclosed in PCT Publication Nos. WO 2013/132418, WO 2013/132419 and WO 2013/132420.
Advantageously, such indirect printing systems allow the distance between the outer surface of the intermediate image transfer member (also called the release layer) and the inkjet print head to be maintained constant and reduces wetting of the substrate, as the ink can be dried on the intermediate image transfer member before being applied to the printing substrate. Consequently, the final image quality is less affected by the physical properties of the substrate and benefits from various other advantages as disclosed in PCT Publication Nos. WO 2013/132345, WO 2013/132343 and WO 2013/132340 by the present Applicant.
The use of transfer members which receive ink droplets from an ink or bubble jet apparatus to form an ink image and transfer the image to a final substrate have been reported in the patent literature. Various ones of these systems utilize inks having aqueous carriers, non-aqueous carrier liquids or inks that have no carrier liquid at all (solid inks).
The use of aqueous based inks has a number of distinct advantages. Compared to non-aqueous based liquid inks, the carrier liquid is not toxic and there is no problem in dealing with the liquid that is evaporated as the image dries. As compared with solid inks, the amount of material that remains on the printed image can be controlled, allowing for thinner printed images and more vivid colors.
Generally, a substantial proportion or even all of the liquid is evaporated from the image on the intermediate transfer member, before the image is transferred to the final substrate in order to avoid bleeding of the image into the structure of the final substrate. Various methods are described in the literature for removing the liquid, including heating the image and a combination of coagulation of the image particles on the transfer member, followed by removal of the liquid by heating, air knife or other means.
Among the problems surmounted by prior art systems was the need to find a balance between opposite requirements. On the one hand, the printing process, including the materials or formulations employed therewith, should allow transiently fixing the aqueous based ink droplets onto the release layer at the image forming station. On the other hand, the same should allow the dried ink film to be fully transferred to the printing substrate at the impression station.
Generally, silicone coated transfer members are preferred, since this facilitates transfer of the dried image to the final substrate. However, silicone is hydrophobic which causes the ink droplets to bead on the transfer member. This makes it more difficult to remove the water in the ink and also results in a small contact area between the droplet and the blanket that renders the ink image unstable during rapid movement and may makes it more difficult to remove the water from the ink, for instance by heating the transfer member. Surfactants and salts have been used to reduce the surface tension of the droplets of ink so that they do not bead as much. While these do help to alleviate the problem partially, they do not solve it.
Another solution proposed in the above-referenced publications of the Applicant to alleviate this problem was to “freeze” the shape of the impinging jetted droplet in the pancake-like form it adopted upon contact, for instance by rapidly evaporating a substantial proportion of the liquid ink carrier at the stage of the image formation onto the transfer member. The rate of such evaporation depending upon temperature, it was generally preferred for that particular purpose to operate the system at elevated temperatures (e.g., above water boiling point and typically up to 160° C.). However, as the vapors of the ink carrier might, over time, affect the print head nozzles, lower temperatures (e.g., above 40° C.) were also considered for the image forming station.
Alternatively, or additionally, the Applicant disclosed conditioning methods and formulations facilitating the desired interaction between ink formulations and materials composing the release layer suitable for the novel process, by pre-treatment of the transfer member ahead of ink jetting. More details on such methods can be found in PCT Publication No. WO 2013/132339.
Without detracting from the importance of these advances, the present inventors have discovered that under some conditions, surprisingly, some of the aforementioned conditioning solutions may deleteriously accumulate on the transfer member on selected areas. Hence, the present inventors have recognized the need for further improvements in release layer conditioning compositions and technologies. The disclosed printing systems for implementing the methods aspects of the disclosure overcome one or more of the problems set forth above and/or other problems of the prior art.